Many corporations and other enterprises are installing private networks to interconnect their facilities to provide voice and data communications. Private networks typically include a number of private branch exchanges (PBXs) or other telephone switches that are linked together. Each PBX, which supports a private network or subnetwork of its own, is typically responsible for telecommunications services in a geographically restricted area, for example, at a particular building or campus occupied by the enterprise. In contrast to a public switched telephone network (PSTN), which is operated by a common carrier, a private network is generally characterized by access limits or restrictions, for example, by designating particular telephones or telephone numbers for internal communications only. As enterprises expand to include an increasing number of offices and sites, they need to link together the PBXs and private networks across wider geographical areas.
FIG. 7(a) depicts one way of linking private networks 700 and 710 together by installing a fixed, point-to-point private line 720 between the two private networks. For purposes of illustration, private network 700 includes a PBX 702 and two (2) telephones 704 and 706, and private network 710 also includes a PBX 712 and two (2) telephones 714 and 716. A fixed, point-to-point private line 720 is coupled between PBX 702 of private network 700 and PBX 712 of private network 710. Accordingly, a user of telephone 704 in private network 700 can call telephone 714 over fixed, point-to-point private line 720 in private network 710.
One drawback for an enterprise in interconnecting private networks at different sites with fixed, point-to-point private lines becomes apparent when the sites are widely separated. For example, to install a fixed, point-to-point private line beyond the boundaries of an office building or campus is often too expensive. When the sites are in different cities or different countries, the cost may become prohibitive. Accordingly, public telephone carriers offer “virtual private networks” to their customers.
A virtual private network is a combination of public and private networks, in which the public portion of the virtual private network carries local and long distance communications between various private networks. In a virtual private network, as illustrated in FIG. 7(b), the customer uses the telephone carrier's public network 730 to interconnect its geographically separated private networks 700 and 710. The network dialing plans, routing schemes, and switches of the public network 730, however, are configured in such a manner that the customer's service appears to be carried over a private network connection. Such a configuration can actually involve the use of several interconnected networks, each of which use may employ a different signaling protocol.
One benefit of private networks over public networks is that they can provide features and services that are unavailable in public networks. A feature is a unit of functionality that is incrementally added to a network to provide an additional service to users of the network. Many PBXs that implement private networks employ various protocols to support a rich set of features. For example, DPNSS, a PBX protocol popular in the United Kingdom, offers Call-Back-When-Free, Three-Way-Calling, Call-Forwarding, and many other special features.
Many private networks, however, implement features that are generally not supported by the protocols used within common public networks and, hence, in common virtual private networks. Protocols or messages that include feature requests not supported by the public network are typically filtered out or otherwise lost at an access point in the public network, because the unsupported feature has no analogue in the signaling message structure of the public network. Although some protocols employed by public networks reserve some space in signaling messages for passing user-defined information, the available space is typically insufficient for directly supporting many features common in private networks. Consequently, calls placed from such a private network across the public network portion are usually limited to the lowest-common denominator, often Plain Old Telephone Service (POTS), thereby destroying the appearance of a wide area virtual private network.
Based on the foregoing, there is a clear need for mechanisms that permit use of advanced calling features in a virtual private network, even when the public telephone network that implements the virtual private network does not support, or have a protocol capable of supporting, features of the private network.
Further, there is a need for a method of transmitting signaling messages over a virtual private network that request advanced calling features not supported by a public network that implements the virtual private network.